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Preparation of GrapheneSelected Topics in Physics: Physics of Nanoscale Carbon
Nils [email protected]
Today there are several methods for the preparation of graphene. In the following some of these meth-
ods will be presented and discussed. They will be compared using specific requirements and allocated
to different purposes. The requirements are e.g. quality and size of the flakes or controllability of the
Graphene is a very special material, since ithas the advantage of being both conductingand transparent. The transparency of a mate-rial normally depends on its electronic prop-erties and requires a band gap. Under nor-mal conditions transparency and conductiv-ity exclude each other, except for a few com-pounds like indium tin oxide (ITO). How-ever, in contrast to ITO, graphene is also flex-ible and capable of withstanding high stress.Therefore it is very attractive for the applica-tion of flexible electronic devices, e.g. touchscreens . Accordingly, there are a lot of ef-forts in order to prepare graphene easyly withthe required properties.
The methods described in this review areevaluated on the bases of different require-ments: On the purity of the graphene, whichis defined by the lack of intrinsic defects,(Quality) as well as on the size of the obtainedflakes or layers (Size). Another aspect is theamount of graphene which can be producedsimultaneously (Amount) or the complexitysuch as the requirement of labour or the needfor specially designed machines (Complex.).One last attribute is the controllability of themethod in order to achieve reproducible re-sults (Control.).
This review is organized as follows: In Sec.2 the exfoliation methods are presented anddiscussed. Following that another type ofpreparation, the growth of graphene, will be
introduced in Sec. 3. Concluding, the differ-ent methods will be compared in Sec. 4.
Basically there are two different approachesto preparing graphene. On the one handgraphene can be detached from an alreadyexisting graphite crystal, the so-called ex-foliation methods, on the other hand thegraphene layer can be grown directly on asubstrate surface. The first reported prepara-tion of graphene was by Novoselov and Gaimin 2004  by exfoliation using a simple ad-hesive tape.
2.1 The Scotch Tape Method
In this micromechanical exfoliation method,graphene is detached from a graphite crys-tal using adhesive tape. After peeling itoff the graphite, multiple-layer graphene re-mains on the tape. By repeated peeling themultiple-layer graphene is cleaved into vari-ous flakes of few-layer graphene. Afterwardsthe tape is attached to the substrate and theglue solved, e.g. by acetone, in order to de-tach the tape. Finally one last peeling with anunused tape is performed.
The obtained flakes differ considerably insize and thickness, where the sizes rangefrom nanometers to several tens of microme-ters for single-layer graphene, depending onthe preparation of the used wafer. Single-
layer graphene has a absorption rate of 2%,nevertheless it is possible to see it under alight microscope on SiO2/Si, due to interfer-ence effects . However, it is difficult toobtain larger amounts of graphene by thismethod, not even taking into account thelack of controllability. The complexity ofthis method is basically low, nevertheless thegraphene flakes need to be found on the sub-strate surface, which is labour intensive. Thequality of the prepared graphene is very highwith almost no defects.
2.2 Dispersion of Graphite
Graphene can be prepared in liquid-phase.This allows upscaling the production, in or-der to obtain a much higher amount ofgraphene. The easiest method would beto disperse the graphite in an organic sol-vent with nearly the same surface energy asgraphite . Thereby, the energy barrier is re-duced, which has to be overcome in order todetach a graphene layer from the crystal. Thesolution is then sonicated in an ultrasoundbath for several hundreds hours or a voltageis applied . After the dispersion, the solu-tion has to be centrifuged in order to disposeof the thicker flakes.
The quality of the obtained grapheneflakes is very high in accordance with the mi-cromechanical exfoliation. Its size howeveris still very small, neither is the controllabil-ity given. On the other hand, the complex-ity is very low, and as mentioned above thismethod allows preparing large amounts ofgraphene.
2.3 Graphite Oxide Exfoliation
The principle of liquid-phase exfoliation canalso be used to exfoliate graphite oxide. Dueto several functional groups like epoxide orhyroxyl, graphene oxide is hydrophilic andcan be solved in water by sonication or stir-ring. Thereby the layers become negativelycharged and thus a recombination is inhib-ited by the electrical repulsion. After cen-trifugation the graphene oxide has to be re-
Figure 1: (a) Solution of graphene in liquid-phase. The flasks contain solutions after cen-trifugation at different frequencies . (b)Scheme of the exfoliation of graphite ox-ide. The graphite gets oxidized and solvedin water. Afterwards it gets reduced tographene .
duced to regular graphene by thermal orchemical methods. It is hardly possible todispose of all the oxygen. In fact, an atomicC/O ratio of about 10 still remains .
The performance of this method is verysimilar to liquid-phase exfoliation of pristinegraphene. Only the complexity is higher,since graphite oxide has to be producedfirst, wich requires the use of several chem-icals. Also the obtained graphene oxidehas to be reduced afterwards, using thermaltreatments or chemicals again . The re-duced graphene oxide is of very bad qualitycompared to pristine graphene, neverthelessgraphene oxide could be the desired prod-uct. Graphene oxide modified with Ca andMg ions is capable of forming very tensilegraphene oxide paper, as the ions are cross-linkers between the functional groups of thegraphene flakes .
2.4 Substrate Preparation
There are different methods for substratepreparation in order to use the dispersedgraphene in a non liquid-phase. By vacuum
filtration the solution is sucked through amembrane using a vacuum pump. As a resultthe graphene flakes end up as filtration cakeof graphene paper.
The deposition of graphene on a surfacecan be done by simple drop-casting where adrop of the solution is placed on top of thesubstrate. After the solvents have evaporated,the graphene flakes remain on the surface. Inorder to achive a more homogeneous coat-ing the sample can be rotated using the spin-coating method in order to disperse the so-lution with the help of the centrifugal force.With spray-coating, the solution ist sprayedonto the sample, which allows the prepara-tion of larger areas.
3 Growth on Surfaces
A totally different approach to obtaininggraphene is to grow it directly on a surface.Consequently the size of the obtained lay-ers are not dependent on the initial graphitecrystal. The growth can occur in two differentways. Either the carbon already exists in thesubstrate or it has to be added by chemicalvapour deposition (CVD).
3.1 Epitaxial Growth
Graphene can be prepared by simply heatingand cooling down an SiC crystal . Gen-erally speaking single- or bi-layer grapheneforms on the Si face of the crystal, whereasfew-layer graphene grows on the C face .The results are highly dependent on the pa-rameters used, like temperatur, heating rate,or pressure. In fact, if temperatures and pres-sure are too high the growth of nanotubes in-stead of graphene can occur. The graphitiza-tion of SiC was discovered in 1955, but it wasregarded as unwelcome side effect instead ofa method of preparing graphene .
The Ni(111) surface has a lattice structurevery similar to the one of graphene, with amissmatch of the lattice constant at about1.3% . Thus by use of the nickel diffu-sion method a thin Ni layer is evaporated
onto a SiC crystal. Upon heating the car-bon diffuses through the Ni layer and formsa graphene or graphite layer on the surface,depending on the heating rate. The thusproduced graphene is easier to detach fromthe surface than the graphene produced bythe growth on a simple SiC crystal withoutNi .
Figure 2: SEM image of graphene on cop-per foil. At several locations on the sur-face graphene islands form and grow to-gether .
The growth of graphene starts at severallocations on the crystal simultaneously andthese graphene islands grow together, asshown in Fig. 2). Therefore the graphene isnot perfectly homogeneous, due to defects orgrain boundaries. Its quality therefore is notas good as that of exfoliated graphene, exceptthe graphene would be grown on a perfectsingle crystal. However, the size of the ho-mogeneous graphene layer is limited by thesize of the crystal used. The possibility to pro-duce large amounts of graphene by epitaxialgrowth is not as good as by liquid-phase exfo-liation, though the controllability to gain re-producible results is given. Also the complex-ity of these methods is comparatively low.
3.2 Chemical Vapour Deposition
Chemical vapour deposition is a well knownprocess in which a substrate is exposed togaseous compounds. These compounds de-compose on the surface in order to grow athin film, whereas the by-products evaporate.
Figure 3: (a)Scheme of preparation of graphene by CVD and transfer via polymer support.The carbon solves into the Ni during the CVD and forms graphene on the surface after cool-ing. With a polymer support the graphene can be stamped onto another substrate, after etch-ing of the Ni layer. Patterning of the Ni layer allows a control of the shape of the graphene .(b) Roll-to-roll process of graphene films grown